Secondary batteries such as lithium secondary batteries (for example, lithium ion batteries) and nickel metal hydride batteries are increasingly, importantly considered as power sources installed in vehicles or power sources of portable terminals such as laptops. In particular, lithium secondary batteries which are light and may provide high energy density may be preferably used as a high-output power source for vehicles, and thus, demand therefor is expected to continuously increase.
In regard to lithium secondary batteries, materials in which intercalation and deintercalation of lithium ions may be performed are used as a positive electrode and a negative electrode active material, a liquid electrolyte is injected after disposing a porous separator between a positive electrode and a negative electrode, and electricity is generated or consumed by oxidation-reduction reaction according to intercalation and deintercalation of lithium ions in the negative electrode and the positive electrode.
In particular, in lithium secondary batteries, various carbon-based material types including artificial graphite, natural graphite, hard carbon, etc., in which intercalation and deintercalation of lithium is possible, have been used as negative electrode active materials. Since graphite among carbon-based materials has a low discharge voltage of −0.2 V with respect to lithium, a battery using graphite as a negative electrode active material exhibits a high discharge voltage of 3.6 V and there are also advantages in energy density of lithium secondary batteries. In addition, long-term lifespan of lithium secondary batteries is guaranteed due to excellent reversibility. However, graphite active materials have a low capacity with respect to energy density per unit volume of an electrode plate due to low graphite density (theoretical density: 2.2 g/cc) upon manufacture into an electrode plate, and problems such as swelling in a battery and consequent capacity reduction, due to side reaction with an organic electrolyte, which easily occurs in high discharge voltage.
In order to address the problems of the carbon-based negative electrode active materials, Si-based negative electrode active materials and negative electrode active materials using oxides such as tin oxides, lithium vanadium-based oxides, lithium titanium-based oxides, having a high capacity, compared to graphite, are being developed and researched.
However, high-capacity Si-based negative electrode materials suffer extreme volume change during charge/discharge and thus particles are split, whereby lifespan characteristics are poor.
In addition, in the cases of oxide negative electrodes, satisfactory battery performance is not exhibited and thus research thereinto are underway. In particular, lithium titanium oxides (hereinafter referred to as “LTO”) among the oxide-based negative electrode active materials exhibit high electricity capacity maintenance ratio and stable lifespan characteristics, e.g., change in a crystal structure does not occur also in an over-charge state. However, there is a problem of battery degradation due to high moisture content in an active material itself.